I had a contractor do some work and he dug a hole eight feet across and nine feet deep. Put all the dirt on my driveway. When done he put all the dirt back into the hole (so he says), now the dirt in the hole is about one foot below the surrounding grade.

I would think it should be above grade level as it has not compacted much. And over time it should setting to be even with the grade. Nothing was added or taken away in the hole, just a repair.

That's messed up. First of all I'm guessing the repair was made next to your home? The dirt should be compacted really well when replaced because water follows the path of least resistance. If that large area doesn't have proper compaction water will start to pool outside of your foundation. I hope you are wrong about the compaction job he did.

Even if he did a really good job of compacting the dirt I doubt he would have lost 45 cubic feet of dirt. Was there a leaking pipe that was being repaired? The only thing I can think of is maybe there was a void from pooling water underground.

Unless someone was around to constantly monitor his work, it woudn't surprise me if he "borrowed" some of your dirt for another project. Actually, it happens more often than you'd think--he's just counting on you being too uninformed to make a fuss about it. I remember a shady residential concrete contractor in NM a few years ago intentionally over-ordering concrete, and then using the excess on nearby "filler" jobs in the same subdivision. Paid a hefty fine when he got caught doing it.

Dirt was taken from the side yard, no plumbing leaks or problems, putting in a sewer cleanout. Composition is mostly clay/gravel here. Excavated with a backhoe and I assume pushed back it with same. Immediately after being done the pile was 1-1.5 feet higher than surrounding yard. But it was next to my driveway and I found a void under the driveway, about 2 feet under, 1 foot deep, and 5 feet wide. After a couple days of rain the pile shrank to be 1 foot below surrounding grade, still having the void under the driveway. With my topsoil/grass nowhere to be seen (I assume somewhere in the filled hole).